This invention relates to cellular mobile radio systems having channels for transmitting analog information between base and mobile stations by transmitting analog modulated radio signals. In particular, the present invention is directed to a method and apparatus for supervising call links or connections and confirming orders on an analog channel between a base station and a mobile station, at the site of the base station.
In cellular mobile radio systems it is fundamental that a base station with an established connection on a radio channel should be able to accurately receive from a mobile station an indication of the radio transmission quality of the connection, e.g., the combination of the down-link and the up-link. Additionally, it is important that the base station be able to be assured that the mobile station has received the base station's orders (e.g., handoff and disconnect orders), and that the base station be able to confirm requests from the mobile station (e.g., release request).
The radio transmission quality is important because, in general, radio communication is only possible when the desired information-carrying radio signals have sufficient signal strength at the receiver and are sufficiently strong in relation to noise and interfering radio signals at the receiver to be distinguished. The minimum strength of course depends on the particular features of the system, e.g., the kind of modulation and receiver used. In order to make sure an established connection may continue on a selected radio channel between a mobile station and a base station, handoff and disconnect processes perform various measurements on radio signals at the intended base and/or mobile stations.
A supervisory audio tone, hereinafter abbreviated SAT, is a continuous tone used for radio transmission quality supervision. During a call, an analog voice channel unit in a base station generates the SAT and adds it to the transmitted speech for transmission in a down-link (to a mobile station), as shown in FIG. 1. The mobile station then loops the SAT back to the base station sometimes adding a Signalling Tone (ST) in an up-link (to the base station), as also shown in FIG. 1. By comparing the SAT signal sent to a mobile station to the SAT signal received from the mobile station, noise in the radio path may be evaluated. The information obtained is used for two main purposes:
(1) deciding whether a handoff or a call release should be performed; and PA1 (2) deciding whether Signalling Tone (ST) information should be accepted. PA1 (1) when detecting a ST as being on, the signalling tone signal is accepted if the SAT strength value indicates that the SAT is on, in accordance with the IS-54-B standard; and PA1 (2) when detecting the ST as being on and the SAT strength value indicates that the SAT is off, the signalling tone signal is not accepted at this stage--however, should the SAT subsequently be detected as being on while the signalling tone is still on, the signalling tone signal is accepted, which is a deviation from the IS-54-B standard. In one embodiment, the measurements of the ST duration are not started until SAT and ST are both on.
When a mobile station confirms certain orders (such as a handoff order), or sends requests (such as a release request), this confirmation is done by sending a special tone, mainly the ST having a specified duration on an analog channel. For this tone to be accepted by the base station, a SAT must concurrently exist under TIA standard IS-54-B for dual mode radio telephone systems which handle both analog and digital radio communications. In the following, a distinction is made between the expressions "ST is on" and "ST is accepted." The phrase "ST is on" means that a radio signal has been observed with the characteristics typical for a signalling tone (ST). However, the TIA standard IS-54-B states that this signal shall not be approved unless a sufficiently strong SAT exists at the same time. If a strong enough SAT exists at the same time, then the system determines that "ST is accepted." Should the ST not be accepted, the base station will have to re-send the order(s) to the mobile station. However, if the ST represents a release request, this release would not be repeated since the mobile station sends it and immediately shuts down. Thereafter, there will be a sequence of presence verification attempts by the base station until it is decided that the mobile station has been released, which is inefficient.
Other problems exist in the prior art systems, particularly the system defined by TIA standard IS-54-B. There are two particular instances where it is difficult, if not impossible, to detect a SAT and/or a ST correctly in the current standard.
The first situation exists when a data message is sent to the mobile station (e.g., a handoff order), or a mobile station sends a data message to the base station (e.g., when acknowledging a power change order). When these data messages are sent, the speech transmission is interrupted. When sending a data message from the base station to a mobile station, the SAT is turned off. When sending a data message from a mobile station to the base station, the mobile does not loop the SAT back to the base station. This is because the combination of SAT and data message would not fit within a 30 kHz bandwidth channel (which is what is available in AMPS), and also there is a risk that the data message and the SAT would disturb each other so that neither of them would be received correctly. The consequence of this is that the SAT is not sent between the stations for a time period. However, if the mobile station has received an order, it confirms it by sending an ST. As seen above, acceptance of the order confirmation or the request (i.e., acceptance of the ST) requires that there actually be a SAT present on the channel in the system defined, for example, by the IS-54-B standard.
In actuality, the SAT may be on again when the ST is sent, but there are limitations in the detectors. For instance, a momentary value of the SAT strength is typically not available because it has to be measured over an interval to be accurate. Also, detectors measuring the SAT are relatively slow to stabilize, taking as much as 100 ms, for example, from activation to when their measurements are sufficiently stabilized.
Measurement results of the SAT as shown in FIG. 2 in a SAT detector are typical. The dotted line shows the actual SAT strength dropping off and starting up relatively instantly, and the solid line shows the lagging more gradual drop off and start up of the measured SAT strength. There may be occasions where the measured SAT strength is not strong enough even though the actual SAT strength is sufficient. The base station consequently will not accept the ST when it otherwise should.
Typical situations leading to this problem are illustrated in FIG. 2a. In the situation shown in FIG. 2a, base station sends a SAT to the mobile station (MS) which receives a SAT and sends it back to the base station (BS). The base station can then detect whether or not the "SAT" is on. In further reference to FIG. 2a, the SAT is turned off for approximately 50 ms when a data message is sent to the mobile station. The mobile station receives the data message for approximately 50 ms and thereafter acknowledges reception of the data message by sending an ST to the base station, which receives the ST and determines that the ST is on. The problem arises by the fast detection of the ST and the slow detection of the returned SAT in the base station, as illustrated in FIG. 2a. This typically occurs when the base station turns on and sends the SAT after the data message is sent, but the "SAT on" is detected in the base station after receiving an acknowledging ST from the mobile station. The mobile station loops the SAT back to the base station, but the base station detects the "SAT on" after receiving the ST. In this system in accordance with IS-54-B, the system will not accept the ST despite it correctly indicating a received data message.
FIG. 2b shows a flow chart which illustrates the procedure according to IS-54-B. As shown in FIG. 2b, at step S2, the SAT and ST are updated while waiting for an ST. At step S3, it is determined whether the ST is on. If the ST is not on, i.e., not detected (not received), the system returns to the update step S2. If the ST is on, then it is determined whether the SAT is on as shown in step S4. If the SAT is on, then the ST is accepted as shown in step S5, and the duration of the ST is measured in S6. If the duration is of an expected length, as determined in step S7, i.e., the measured length is within an expected interval, then the ST is interpreted as a message from the mobile, at step S8. If the length of the measured duration of ST is too short or too long, the received ST is regarded as not a message from the particular mobile station, as shown in step S9. This is the normal course of events for which the IS-54-B standard is appropriate.
However, at step S4 if it is determined that the SAT is not on (while the ST is on), the system waits (step S10) until the ST is off. This step indicates that if the ST goes on during the time when the SAT is off for a short while, the ST will not be accepted, although the SAT and the ST may simultaneously be on long enough for the ST to be regarded as a request/acknowledgment from the mobile station, should it only have been accepted.
Once it is determined at step S10 that the ST is now off, the system according to IS-54-B, returns to step S2 to update the SAT and ST.
The problem not solved by IS-54-B is if the SAT is off for a short period during which the ST is detected. For this situation, IS-54-B does not provide an appropriate solution. This could be compared to the fact that once ST has been accepted, the IS-54-B standard does not require that SAT be present during the entire period when the duration of the ST is measured, discussed in more detail below.
Previous solutions include removing the SAT strength criterion when accepting the ST during and for a time period immediately after a data transfer to or from the mobile station. This is not a particularly appropriate solution since there is no guarantee that the SAT strength condition was acceptable to begin with.
The SAT strength value is also used by the base station for other purposes than SAT-ST qualification. As mentioned before, it is also used as a quality measure when evaluating whether or not a handoff or call release should be performed. When the SAT is turned off for the sending of a data message, the measured SAT strength will drop slowly, as shown in FIG. 2. When the SAT is turned on again after the sending of the data message, the measured SAT strength will rise slowly back to the required value.
Another problem with the prior art is that the IS-54-B standard is rigid in the manner in which it will recognize SAT and ST as being on or off simultaneously and, consequently, in the manner in which it will accept signalling tones. In actual situations, variations in the measured SAT are possible and oscillations become apparent that may or may not correspond to actual variations. These oscillations can rapidly cross the threshold between on and off determinations, leading to inconclusatory decisions as to whether it is on or off.
It is not satisfactory to look at a momentary SAT value when determining whether to accept a ST of sufficient strength. This is because, if only momentary values are examined, the system will be vulnerable to oscillations in SAT strength crossing the threshold between SAT on and off determinations, depending on the speed of the detectors which require some period of time in which to measure the SAT value. If the SAT value is taken over an interval longer than required by the detectors, it is difficult to determine what length interval should be used. If the interval is too short, the system will be vulnerable to rapid oscillations. On the other hand, if the interval is too long, fast oscillations will not be noted when they should be. The interval ought to be of the same order as the length of the ST, but this still leaves a broad range. Knowledge of the actual system and the environment into which it is placed can be used to decide the actual value to be used as a practical solution. But even when the measurement interval is carefully determined under these criteria, there would be occasions when the ST value is not accepted even though the SAT is present, albeit at a strength oscillating such that the measurement value repeatedly passes a threshold value used to determine if the SAT is on or off.
The system is attempting to detect an ST signal of an expected duration. To be certain that the system is detecting an ST signal from the correct mobile station, the system requires that the base station receive a strong enough SAT signal from the mobile station. In order to detect the SAT signal, it must be measured over an interval. To reiterate the above, if the interval is too short (much shorter than the expected length of the ST), more information than necessary about the SAT variations will be provided to the system. Thus, the decision making process based on SAT values would become more complicated because of the frequent variations of the measured SAT strength. If the interval is too long (much longer than the length of the ST), the decision making based on the SAT values would be too insensitive. An ST could be accepted although the ST was off during the interval when the SAT was received, or vice versa, simply because the measured SAT value was averaged over too long a period.